The present invention relates to electrochemical cells and primarily to cells having a lithium metal or lithium alloy anode.
Thermal cells are primary electrochemical cells having an anode, a cathode and an electrolyte that is solid and non-conductive at ambient temperatures. The cell is activated by providing sufficient heat to melt the electrolyte which thereby becomes conductive. Thermal batteries are made up of a plurality of thermal cells. These batteries conventionally include a heat source, generally an ignitable, exothermically reactive chemical charge or pyrotechnic.
Thermal batteries are normally used to generate a single continuous energy output, once activated. The duration of the output may vary from a few seconds to an hour, depending on the battery design and construction. These batteries are frequently used as power sources for guided missiles and other weapons systems.
A variety of electrochemical systems are known for use in thermal cells. Known anode materials include the alkali metals and their alloys, alkaline earth metals and their alloys and aluminum. The electrolytes are generally mixtures of alkali metal halides, commonly a eutectic mixture of LiCl and KCI melting at about 352.degree. C., although other fusible salt mixtures have been used, such as alkali metal thiocyanates, as well as solid electrolytes, such as the alkali metal aluminum chlorides. Suitable active cathode materials that are reduced in the electrochemical cell reaction include phosphates, metal oxides, borates and chromates. The terms cathode, depolarizer and depolarized cathode are frequently used in discussions of battery technology. Herinafter, only the term "cathode" will be used, and will be understood to embrace the meanings carried by any of these terms.
Lithium is an attractive anode because of its reactivity, light weight and high voltage (between 1.6 and 3.6 volts, depending on the cathode material). The advantages of lithium include high energy density, flat discharge characteristics, and good shelf life.
Known thermal batteries usually employ the following cell construction (although other cell constructions are known). The anode is provided by a layer of lithium either in the form of the pure metal held in a support (since the metal is liquid in the thermally activated state) or in the form of an alloy, e.g., with boron, silicon or aluminum. The electrolyte is a pressed powder pellet comprising a eutectic mixture of LiCl and KCl, or of LiF, LiCl and LiBr. The electrolyte is admixed with an inert binder such as MgO to immobilise it when molten.
Several cells are arranged in a stack, each cell having its own pyrotechnic pellet. The stack is assembled in a dry atmosphere and is enclosed by material providing a thermal blanket. The enclosed stack is then hermetcially sealed within a steel case.
The common cathode materials known to be used in thermal batteries are CaCrO.sub.4, V.sub.2 O.sub.5 and FeS.sub.2. Of these, CaCrO.sub.4 and V.sub.2 O.sub.5 are soluble in molten LiCl/KCl eutectic. Therefore, a portion of the active cathode material must be expended in the formation of a separator layer to prevent self discharge. Cathodes of these materials are also subject to concentration polarization in the course of discharge.
FeS.sub.2 is insoluble in the LiCl/KCl eutectic, hence avoiding the two difficulties mentioned above. FeS.sub.2, however, possesses limited thermal stability, undergoing thermal decomposition in LiCl-KCl eutectic at about 550.degree. C.
Conventional room or ambient temperature cells having lithium anodes are also known. Such cells have the advantages given previously.
Nonaqueous solvents are used as the electrolyte because of the solubility of lithium in aqueous solutions. Organic solvents, such as acetonitrile and propylene carbonate, and inorganic solvents, such as thionyl chloride, sulfuryl chloride and sulfur dioxide, are typical. A compatible solute is added to provide the necessary electrolyte conductivity. A number of different materials, e.g., sulfur dioxide, carbon monofluoride, vanadium pentoxide, manganese dioxide, copper sulfide, and the like, have been used as the active cathode material.
It is therefore an object of the present invention to provide a novel cathode material for thermal cells.
It is another object of the present invention to provide a novel cathode material for conventional lithium cells.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the invention.